What is the smallest dollar amount for which a seller would be willing to sell an additional unit, typically equated to marginal cost?

The smallest dollar amount for which a seller would be willing to sell an additional unit corresponds to the concept of marginal cost, which represents the additional cost incurred by producing one more unit of a good or service. The seller's reservation price is defined as the minimum price at which a seller is willing to sell a good, effectively capturing the seller’s cost of production and the marginal cost.

When a seller assesses whether to produce an extra unit, they will compare the price they could receive for that unit against their own marginal cost. If the price meets or exceeds their marginal cost, the seller is incentivized to increase production. Therefore, this reservation price is fundamentally tied to the seller's willingness to sell based on their costs.

The other choices do not accurately capture this concept. The buyer's cost pertains more to the amount a buyer is willing to pay, while the marked price often refers to the selling price that a good is advertised for. Market equilibrium price indicates the price at which quantity supplied equals quantity demanded in the market, rather than the specific price at which a seller is willing to sell an additional unit. Thus, the seller's reservation price is the most precise term that fits the definition provided in the question.